Usually semi-autogenous mills (SAG) and bowl mills which are designed for appropriate application are used. These mills include a rotating cylinder which is filled with ore which is to be milled or pulverized. Since the cylinder rotates around an axis which essentially lies horizontally, the material (charge) will be lifted in the mill and subsequently falls back onto the material left below on the floor of the mill. In this connection gravitational forces in particular are exploited. The impact of the lumps of ore and also the friction within the circulating charge causes the ore to be broken up.
In order to increase the milling performance, in a few mill variants steel balls are additionally added to the charge. The agitation or raising of the charge material is supported during rotation by what are known as liners, which are embodied in the shape of paddle-like structures on the inner mill wall.
For optimum control purposes it is useful to be able to measure characteristic variables of the milling methods used. The following are examples of such variables:                The volumetric charge level of the mill,        The geometrical position of the charge,        The weight of the charge,        The amount of steel balls in the mill,        The amount of water in the mill,        The number of impacts on the liners per unit of time and        The size distribution of the lumps of ore in the mill.        
In order to minimize the enormous energy consumption of the mill or to maintain a constantly high throughput or in order to reduce the number of particles striking the inner mill wall (liner impact), the knowledge of the above data of the milling plants is useful. Because of the milling geometry and the harsh environmental conditions it is not readily possible, in certain conventional milling plants, to measure the above-mentioned method variables during operation for example.
Different methods are known in the art to determine the overall weight of the material in the mill for example. A simple form includes evaluating the oil pressure in the mill bearings. A further way is by measuring the power consumption [1].
In the art in accordance with [2] the overall weight of the mill charge is determined by its influence on the initial mass inertia moment of the mill. The moment can be derived from the reaction of the mill speed to a small change in the torque.
In publication [3] an electromagnetic source within the mill and a receiver on the outside of the mill are used to measure the amount of ore and of steel balls in the mill and also for measuring the liner wear.
A classical and currently generally used way of determining the charge of a mill with a quantitative measure is what is known as the “Hardinger Electric Ear Method” [4]. If the loading of the mill is too small, numerous impacts on the liners will be recorded and generate loud noise. The “Hardinger Electric Ear” uses a microphone on the outside of the mill to measure the amplitude of the sound being produced.
The acoustic amplitude recording can be searched for amplitude peaks, from which the precise number of impacts on the liners can be deduced. This system is known as an “Impact Meter” [5] (crushing or impact measuring device).
Acoustic detection has been further developed by [5, 6]. In these methods a wireless microphone is attached to the mill sleeve or mill outer wall, in order to measure the breakage/impact noise inside the mill which is generated in the vicinity of the position of the microphone. Since the microphone rotates together with the mill, this “sees” all the positions in a mill cross-section. The geometrical position of the charge can be derived from the difference between the sound amplitudes.
In accordance with publications [7, 8] the use of electrical control variables of a load commutated inverter (LCI) drive is proposed in order to estimate the current load of the mill in relation to the current and in order to detect particular error states of the LCI drive. The method appears to be particularly designed for LCI drives.
The said methods include a little information about the milling status. However no measurement is possible for example for the lump size distribution and also for reliable measurement of the proportions of steel balls, ore and water during operation.